Radical reactions with selenium precursors

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

Acyl(phosphido) complexes, with Pt(II), 8, 458 Acyl radicals, via selenium precursors, 9, 477 Acylsilanes, applications, 9, 319 Acylstannanes, preparation, 3, 822-823 l-Adamantyl-2-pyridyl amido complexes, with Zr(IV), 4, 782 Adaptive quantum control, for selective bond cleavage, 1, 247 Addition reactions... 

Other methods for the preparation of selenium-containing molecules for radical reactions have been used as well. In Scheme 8 the radical cyclization of compound 44 to the tetrahydrofuran derivative 45 is shown as a crucial step in natural product syntheses. Mixed acetal can also be used for the generation of radicals. 1-Alkoxy and 1-aminoalkyl radicals can be generated from 0,Se- and N,Se-acetals in a direct way. The corresponding 0,Hal- or N,Hal-mixed acetals are instable and this approach is rarely used. But the selenium-containing mixed acetals can be isolated and purified and are excellent radical precursors.241 For example, acetals such as 139 can be converted into the mixed 0,Se-aeetals by reaction with diisobutylaluminum phenyl selenide. The mixed acetal 140 is then used under radical reaction conditions for the efficient formation of new carbon-carbon bonds (Scheme 36).242... 

An efficient way of introducing selenium in a radical precursor is the use of selenium containing building blocks. The selenides are particularly appropriate when the reaction sequence involves reaction steps that are incompatible with halides or when the corresponding halides are not stable. In Eq. (7), preparation of the selenenylated alkenyl sulfoxide 35 by alkylation of malonate 33 with the bromide 34 is described [18]. This procedure is not feasible with the corresponding halide due to cyclopropane formation via intramolecular alkylation. Radical cyclization of 35 in a 5-exo mode affords, after elimination of the phenylsulfinyl radical, the methylenecyclopentane 36 in good yield and excellent enantioselec-tivity. 

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